Optical waveguides or optical fibers have been widely used in communications, having become the preferred wide band communication media. Due to providing an increase in transmission capacity, the single-mode optical waveguide has received special attention.
During tha manufacturing phase of an optical waveguide, especially those made out of silicate glass and, more specifically, during the transformation of the preform, the matrix glass is submitted to softening temperatures over 2000.degree. C., tensed and rapidly cooled generating, radially, internal stresses that give rise to structural defects that increase the waveguide optical loss (Ainslie, B. el al., 25, vol, 12, no. 2 British Telecom Technology, 1984)
The levels of internal stress on the optical waveguide increase with the stress applied during the drawing of said waveguide, the region of greatest internal stresses being the region between the core of the waveguide and the adjacent optical cladding region, since the thermal expansion coefficient and viscosity in these regions differ, originating stresses between these regions. Furthermore, these stress levels depend on the concentration of the dopants used in the glass composition of the optical waveguide, such as P.sub.2 O.sub.5, F and GeO.sub.2, during the construction of the refractive index profile.
It is known that when a certain region of an optical waveguide preform, with a lower viscosity than that of the cladding glass, is submitted to a drawing process, this region is put under compression. One of the known effects of submitting an optical waveguide to compression is the refractive index variation: the higher the level of compression, the higher the increase in refractive index (photoelastic effect).
For a given refractive index profile it is possible to control the viscosity and the thermal expansion index and, consequently, the internal of each of the core and cladding regions, through the adequate use of the materials during the chemical deposition step. This adequate combination of materials involves, for example, the control of the level of thermal expansion which will compensate or simply minimize the above-mentioned radial compression. This control of the thermal expansion can be obtained by means of a controlled incorporation of dopants, such as GeO.sub.2, F and P.sub.2 O.sub.5, together with the matrix glass SiO.sub.2.
In a known solution, utilizing waveguides with a step-index profile, a decrease of the residual stresses after the manufacture of the optical waveguide is obtained during the chemical processing of the waveguide, controlling the use of fluorine and germanium in the core and the cladding of the waveguide, with SiO.sub.2. This composition makes it possible to build structures where the viscosity is the same along the entire cross section of the waveguide, strongly reducing the stress problem. However, this methodology is only applicable to some optical waveguide manufacturing techniques.
Further to the problem of controlling internal stresses, other parameters relevant to the operation of optical waveguides must be observed, such as, low attenuation and chromatic dispersion, the latter associated to the form of the refractive index profile of the core.
In the efforts to broaden the single-mode optical waveguide bandwidth, the reduction in the chromatic dispersion of these optical waveguides at the operation region of the optical sources is of great importance.
Although the wavelength of 1330 nanometers presents, almost naturally, a low chromatic dispersion, the point of minimal attenuation of optical waveguides is at the spectral region of 1550 nanometers. Hence, solutions have been developed for obtaining optical waveguides presenting low chromatic dispersion at this wavelength. In order to obtain a chromatic dispersion near zero at this region, a triangular type of core refractive index profile is used, whereby a gradual variation of said index is obtained between the center of the core of the optical waveguide and the optical cladding. This profile is obtained by gradually doping germanium onto the core material of the optical waveguide.
Another solution so that the optical waveguides present low chromatic dispersion at the 1550 nanometer region is to reduce the diameter of the core. However, these solutions reduces the cutoff wavelength, that is, of the wavelength over which the waveguide presents a single light guidance mode, becoming a single-mode waveguide. This reduction in the cutoff wavelength causes an increase in attenuation at the 1550 nanometer region because a very extense distribution of the electrical field within the waveguide occurs, considerably extrapolating the optical waveguide core, making it sensible to effects occurring at its outer surface, such as micro and macro deformations by extraneous agents in situations at which the optical waveguides are normally subjected, such as processes to apply the primary and secondary coatings, processes to arrange them in reels, processes to arange the waveguide in an optical cable, processes to install the optical waveguides and/or cables in ducts or directly buried or as aerial cables.
The reduction in the cutoff wavelength shifts the region of curve of attenuation variation per wavelength, where there is a considerable increase in attenuation, to the 1550 nanometer wavelength region. The reduction in the attenuation levels due to micro and macrocurving effects have been obtained by increasing the cutoff wavelength through modifications of the basic triangular refractive index profile. Experimental essays show that, in a 1550 nanometer triangular profile single-mode waveguide, the attenuation induced on the waveguide decreases very fast in response to an increase in the cutoff wavelength Smolka, F. M., X Outside Plant Seminar of the Telebras System, 1993!.
Different ways to increase the cutoff wavelength without losses in other features are known in the art. In one of these solutions, to triangular profile is added a concentrical high refractive index region, radially separated from the central profile (Bhagavatula, V. A. et al., Proceedings of the Optical Fiber Conference, 94, 1985). However, the cutoff wavelength of these optical waveguides is difficult to determine (Shah, V. et al., Proceedings of the Optical Fiber Communication Conference, 77, 1988).
In another known solution, the increase in the cutoff wavelength is obtained through the radial variation of the chemical composition of the materials that constitute the core and resulting in a double-core refractive index profile wherein a parabolical profile is superposed to a rectangular profile (Ohashi. N. et al., Chronicals of the First Optoelectronic Conference, p 22, 1986). However this solution does not solve the internal stress problems due to manufacturing.
Hence, the main object of the invention is to present an single-mode optical waveguide with a minimal and controllable internal stress due to manufacturing effects and to variations of the refractive index between the optical core and cladding.
A specific object of the invention is to present a single-mode optical waveguide which permits altering the viscosity and thermal expansion coeficients of the material out of which said optical waveguide is produced, resulting in a release of the stresses induced during the drawing of the optical waveguide.
Another object of the invention is to present a single-mode waveguide which, apart from the above-mentioned advantages, permits the obtention of a null chromatic dispersion in the 1550 nanometer spectral region and an increase in the cutoff wavelength, without affecting the signal attenuation in this region.
An additional object of the invention is to present a single-mode optical waveguide with the above-mentioned features and that further presents low sensibility to micro and macrobending effects that the optical waveguide is subjected to, during handling and use.